1. Technical Fields
The present invention relates to an illumination apparatus configured to illuminate, for example, a light-modulating element, and a projector configured to project and display an optical image formed on an image forming element on a screen by a projection lens and, more specifically, to a configuration of an illumination apparatus having a plurality of light source units.
2. Related Art
As one of apparatuses which are capable of displaying a large-screen video, a projector configured to illuminate a small-sized image forming element which forms an optical image according to video data by a light from an illumination apparatus and display the optical image on the screen or the like by a projection lens has become commercially practical. In the projector of a type described above, upsizing of a screen, increase in luminance, and reduction of luminance nonuniformity or color nonuniformity in a projected image is strongly required, and a high-performance illumination optical system for realizing these requirements has now been in development.
In order to achieve increase in luminance of the projected image, a projector having a plurality of light source units (light source lamps) has become commercially practical. For example, in an illumination apparatus (see FIG. 1 in Japanese Patent No. 3581568 (Patent Document 1)) of a projection-type display apparatus (projector) disclosed in Patent Document 1 shown below, lights from two light source lamps are condensed by respective oval reflectors, and reflected by reflecting surfaces of a reflection prism arranged at a condensed position in the direction toward an integrator to form a combined light which is emitted in substantially one direction, so that increase in quantity of emitted light is achieved. The lights emitted from the optical source lamps are condensed with a smallest luminous flux diameter at a second focal point of the oval reflector, and a light source image is formed on the reflecting surfaces of the reflection prism. In this configuration, positions of the two light source images formed on the reflection prism is closer to an illumination optical axis than the actual position of the light source lamps, and hence it is equivalent to an arrangement of the two optical source lamps close to the illumination optical axis. Therefore, a light including two lights from the two light source lamps combined substantially into one can be generated while restraining a divergence of the angular distribution in the illuminating light.
Here, since most of optical elements such as the image forming element, the polarizing element, and the projection lens have an angle dependency with respect to an incident light as optical characteristics, a combined light having both a small luminous flux diameter and a narrow divergence of the angular distribution is desired. In order to do so, what is important is to reduce the luminous flux diameters of converged lights formed by the respective oval reflectors (in other words, the sizes of the light source images) as much as possible, and to arrange the respective light source images at a position as close to an apex of the reflection prism as possible. However, the more decreased the luminous flux diameters of the converged lights, the larger the light energies concentrated locally on the reflecting surfaces of the reflection prism becomes. Consequently, there arise problems such that reflecting films formed on the reflecting surfaces are deteriorated by locally generated heat or heat strain and the reflectance ratio is lowered, and that the reflecting surfaces by themselves are easily damaged and hence a high light output cannot be obtained stably as the illumination apparatus over a long period.
In order to avoid these problems, measures such as (1) arranging the reflecting surfaces at positions apart from a position where the luminous flux diameter of the converged light becomes minimum in the direction of the optical axis and preventing local concentration of the light energy, (2) limiting the intensity of the lights incident on the reflecting surfaces from the light source lamps, and (3) cooling the reflecting surfaces strongly, are contemplated. However, in the measure described in (1), since the converged light is positioned at a position apart from the apex of the reflection prism, the luminous flux diameter of the combined light reflected from the reflection prism and emitted in substantially one direction is increased and hence the angular distribution is widened, whereby the illuminating efficiency is lowered. In the measure in (2), since the quantity of the emitted light of the illumination apparatus is lowered, increase in luminance of the projected image as an original object is impaired. In the measure (3), a cooling device is upsized and tends to generate a large noise correspondingly, so that usability is lowered. In this manner, any of these measures cannot solve the problems fundamentally.